Generally, compared to a scheme of directly estimating a frequency domain channel in a received signal, a Multiple-Input Multiple-Output (MIMO) orthogonal frequency division multiplexing (OFDM) system initially estimates a time domain channel and then finally estimates a frequency domain channel using the estimated time domain channel. The MIMO OFDM system can reduce the noise affect and also can effectively perform interpolation between pilot subcarriers to improve the channel estimation performance. Channel estimation schemes can be generally classified into a disjoint channel estimation scheme and joint channel estimation scheme.
The disjoint channel estimation scheme considers, as interference, other signals except for its own signal to estimate a channel. For example, on the assumption that a number of multi-paths of a time domain channel is P, when the conventional disjoint channel estimation is adopted, P×P matrix inversion is required for each OFDM symbol.
On the assumption that a number of transmit antennas is Nt, the joint channel estimation scheme simultaneously estimates channels corresponding to all the transmit antennas. Therefore,NtP×NtP
matrix inversion is required for each OFDM symbol. Generally, since P>50, it is very difficult to embody the joint channel estimation scheme. In the conventional art, proposed was a scheme of canceling an interference component, caused by another antenna, using interference estimation obtained from an iterative reception process to improve the channel estimation performance. However, since the conventional scheme does not adopt a time domain channel estimation process, the performance may be significantly deteriorated when frequency selectivity or time selectivity is large.
When mixing the conventional schemes, the performance of mixed scheme can be similar to the performance of the joint channel estimation scheme and the complexity thereof may be similar to the complexity of the disjoint channel estimation scheme. However, since the mixed scheme also requires P×P matrix inversion for each OFDM symbol, there still remains a problem that the complexity thereof is very high.
In the case of an initial channel estimation scheme of estimating a channel using a pilot symbol, a channel estimation value in a previous iteration for interference cancellation does not exist and thus the interference cancellation becomes impossible. Specifically, the only solution is to apply the conventional joint channel estimation scheme.
In the case of a conventional noise variance estimation scheme, when per-subcarrier estimation is performed, the performance can be deteriorated due to a noise component.